The invention relates to an optical beam scanning device and a diaphragm device. For example, the invention relates to the optical beam scanning device which is capable of adjusting light quantity distribution of laser beams reaching a scanning surface, and the diaphragm device for uniforming light quantity distribution of a light flux section of emitted light vertical to a light advancing direction.
Conventionally, optical scanning devices adopt roughly two kinds of systems for scanning light from light sources on recording media.
One of them is a system for allowing laser beams narrower than an image width of a reflecting surface of a rotating polygon mirror enter in a horizontal scanning direction and scanning them (hereinafter, “under field type”). The other one is a system for allowing laser beams wider than the image width of the reflecting surface of the rotating polygon mirror in the horizontal scanning direction enter and scanning a part of the laser beams cut off by the reflecting surface (hereinafter, “over field type”).
In order to solve non-uniformity of light quantity distribution on recording media in the over field type optical beam scanning devices, conventionally, the following technique for correcting light quantity distribution on recording media is present.
Japanese Unexamined Patent Application Publication No. 11-218702 discloses that in the over field type optical beam scanning device, an aperture plate is provided between a collimating lens and a concave lens and an opening shape of the aperture plate is such that a length in a vertical scanning direction on both end positions in a horizontal scanning direction is larger than a length on a center position in the horizontal scanning direction.
Japanese Unexamined Patent Application Publication No. 2001-108926 discloses that in an over field type optical beam scanning system, a slit member having a slit is provided on a halfway optical path between a light source and a polygon mirror and a slit shape is determined so that a light quantity for exposing positions on a surface to be scanned (on a recording medium) from a scanning start position to a scanning end position in one scanning becomes approximately constant.
Japanese Unexamined Patent Application Publication No. 2001-125033 discloses that in an over field optical beam scanning system, a diaphragm, in which an aperture diameter in a vertical scanning direction on at least one peripheral portion of a condensing lens system is wider than that at a center of the condensing lens system, is provided between a laser light source and a polygon mirror.
Japanese Unexamined Patent Application Publication No. 2002-023092 discloses that in order to uniform light quantity distribution on a recording medium in an over field type optical beam scanning device, a positional relationship between a rotating polygon mirror and a laser beam is set as follows. When an effective reflecting width of a reflecting surface of the rotating polygon mirror becomes small, a part of a laser beam with large light intensity distribution is reflected, and when the effective reflecting width of the reflecting surface becomes large, a part of the laser beam with small light intensity distribution is reflected.
In an actual laser beam, however, light intensity in Gaussian distribution disperses to a certain degree. In Japanese Unexamined Patent Application Publication Nos. 11-218702 (1999), 2001-108926 and 2001-125033, however, since a correcting amount of the light quantity distribution is fixed, these techniques cannot cope with dispersion of a laser beam.
In Japanese Unexamined Patent Application Publication No. 2002-023092, the light quantity distribution can be corrected in a device in which a change in the effective reflecting surface always increases or decreases (so-called over field type optical beam scanning device for grazing incidence). The light quantity distribution can be, however, corrected in only one of the cases where the change in the effective reflecting surface increases and decreases (so-called over field type optical beam scanning device for front incidence). Further, in Japanese Unexamined Patent Application Publication No. 2002-023092, a light quantity on a recording medium is reduced further towards a right end of a graph in the embodiment, and thus the light quantity is not completely corrected.
Further, in Japanese Unexamined Patent Application Publication No. 2002-023092, a displacement unit which enables an incident position of a laser beam to be displaced in a light scanning direction is provided so that adjustment is possible. Since a total number of parts increases and a light source is moved, however, locating of an optical axis becomes a problem.